1. Field of the Invention
This invention relates to apparatus for testing glassware for defects and more particularly to a method and apparatus for testing the resistance of glassware to thermal shock and impact loads.
2. Description of the Prior Art
Commercial glassware and specifically glass bottles are tested to prevent ware of inferior quality from leaving the factory and for the purpose of effecting any changes in the glass or in production. During the bottling, handling the packaging stages glassware is subjected to both thermal and impact stresses. For example, in the pasteurizing process bottles are subjected to a long-enduring temperature gradient which may be rather low. Then on other occasions a severe temperature gradient may be encountered. A beverage bottle, for example, taken from a warm room and placed in an ice chest or a tank of ice water is subjected for a period of time to a severe temperature gradient. A mason jar which is initially at a little above room temperature may be poured full of boiling fruit juices. Bottles are subjected to a variety of impact loads such as when labels are forcibly applied to flat panels of a bottle and when bottles are case packed. Consequently, glass bottles must adhere to certain minimum requirement insofar as resistance to thermal and impact loads.
The containers which do not meet the minimum requirements must be detected during the manufacturing process prior to filling and handling the bottle. It is the conventional practice to test the thermal endurance of glassware by either setting up tension in the outside wall of the ware or setting up compression in the inside wall. It is well known to cause tension in the outer walls of glassware by dipping a hot bottle into a cold liquid and by pouring a hot liquid into a cold bottle. When hot liquid is poured into a cold bottle the tension that is applied is indirect. The hot liquid in contact with the inside of the article sets up compression in the inner walls thereby placing the outer walls in tension.
A well known thermal tester is disclosed in U.S. Pat. No. 2,167,185 whereby a bottle is subjected to hot and cold baths to induce fracture at defective points in the bottle. A basket is loaded with bottles and immersed in a tank of hot water to a point where the bottles are completely filled with water. The bottles remain in the tank of hot water for approximately five minutes. Thereafter the basket is transferred to a cold water tank where it remains for thirty seconds. The bottles are immersed in the cold tank to a point below the top of the bottle. The basket is removed from the cold water tank, and the individual bottles are visually inspected to detect any breakage.
U.S. Pat. No. 2,764,015 discloses a device for detecting a defect in a glass container by controlled differential heating. The interior surface of the container is rapidly heated in about one second to create a momentarily high tensile strength in the outer exterior surface. The rapid heating of the interior surface without corresponding heating of the exterior surface results in the creation of temporary stresses which will break the container at the point or points of location of the external surface defects. The interior surface of the container is quickly heated by hot gasses, such as steam or compressed air, to a high temperature with the result that the exterior surface is subjected to high tensile stress causing breakage if a defect should exists.
Another well known method for testing glassware for both thermal and mechanical weaknesses is disclosed in U.S. Pat. No. 2,301,316 where articles to be tested are conveyed through a fluid bath heated to a desired temperature. The fluid bath is maintained at a temperature of 500.degree. F. so as to subject the articles to thermal shock of sufficient magnitude to destroy the articles which are unfit for use.
While thermal endurance tests can reveal defects in different parts of a container, such as flaws in the bottom parting line, lower sidewall, or baffle, impact tests are applied at only one point. Thus only limited information on the impact endurance of a bottle can be obtained. Commonly known methods for testing impact resistance are dropping weights on the object, using a heavy pendulum, or squeezing the bottle. Because of the different sizes and shapes of bottles these tests do not provide consistent information unless the tests are carefully applied at the same part of the bottle.
The known method of thermal shock testing by submerging bottles in one tank and then transferring them to a second tank of a different temperature is a time-consuming task that interrupts the production process. The test must be manually performed and if the test is successful in producing a fracture the fracture must be detected upon visual inspection. If the visual inspection is not conducted diligently, then the flaw will go undetected.
A container may remain intact as a result of a thermal shock test in which a partial fracture has occurred. The container will have been substantially weakened, but the flaw will not be visibly detected. As a result there will be no advance warning of an upcoming problem.
Overall the realiability of both the known impact and thermal shock test methods are dependent on the diligence of an operator to visually examine the test sample to determine if breakage has occurred. Therefore there is need for a method and apparatus for detecting substandard resistance of glassware to impact and determining thermal shock.
The known methods are time consuming and labor intensive. Separate tests have to be conducted for determining shock and impact endurance. In many cases the glassware to be tested is removed from a production line and tested while production continues. Consequently if flaws are detected the line must be shut down and all products retrieved even though they may have continued onto subsequent stages. Therefore there is need for a method and apparatus for testing thermal and impact endurance of glassware in an automated fashion to the extent that sample bottles are selected, tested and examined for breakage without operator assistance.